The present invention pertains to a method for converting a composition into thermoplastic polyurethane, said composition comprising polyurethane resin. The invention particularly concerns articles obtained by shaping the thermoplastic polyurethane obtained by this method. These articles are used as eyewear lenses such as sunwear, ophthalmic lenses, safety glasses, shields and goggles.
The material used to produce eyewear lenses must be optically clear. In addition, the finished product must meet specifications with regards to the color of the product. In particular, the color must remain stable during the life of the product. Certain materials, among them thermoplastic polyurethanes, are not suitable for use in eyewear applications due to a high initial Yellowness Index (YI) and/or due to an increasing YI because of its instability when exposed to UV radiation (e.g. from sunlight).
The rigid thermoplastic polyurethane resin ISOPLAST was first introduced and manufactured by Dow Chemical Company. In the ISOPLAST resin family, the commercially available thermoplastic polyurethane ISOPLAST® 301 produced by Lubrizol Advanced Materials is an injection molded transparent polyurethane derived from 4,4′-methylene diphenyldiisocyanate (MDI) and a mixture of 3 diols which are 1,6-hexanediol, 1,4-bis(hydroxymethyl)cyclohexane and polytetramethylene glycol. As the raw material MDI is pale yellow, the ISOPLAST® 301 resin is tinted with yellow with a measured Yellowness Index of 6 for a 3.2 mm thick injection molded sample based on ASTM D 1925. Thus ISOPLAST® 301 has both a high initial YI and an increasing YI as a function of time.
Nevertheless, in view of its high toughness and its chemical resistance, this material ISOPLAST® 301 material would be of interest for use in eyewear applications, if it were transparent.
There is therefore a need for a method for producing thermoplastic polyurethane such as ISOPLAST® 301, which can be used in eyewear applications, overcoming the limitations of the known thermoplastic polyurethanes, without compromising its other optical and mechanical properties. Advantageously the present invention discloses a method that addresses the issues discussed above. These and other advantages are achieved in accordance with the present invention as described below.